Trevor Hall, regional officer at Unite, said: “Unite will be keeping a careful eye on developments to ensure that staff being transferred to the new owner once the deal is completed do not suffer any detriment to their jobs.

“While there have been no indications so far of job cuts, the union will be proactively working with GSK and Suntory to ensure this remains the case.”

GSK said Lucozade and Ribena, which date back to 1927 and the 1930s respectively, will be in “good hands” with Suntory.

GSK one of the world’s biggest pharmaceutical companies, said it was selling the brands after deciding to focus on a “core portfolio” of consumer healthcare brands with potential in emerging markets, such as toothpastes Sensodyne and Aquafresh as well as Panadol, and Nicorette.

David Redfern, GSK’s chief strategy officer, said: “Lucozade and Ribena are iconic brands that have made a huge contribution to GSK over the years, but now is the right time to sell them as we increase the focus of our consumer healthcare business and execute the delivery of our late stage pipeline of pharmaceuticals and vaccines.

“We believe the future of Lucozade and Ribena is in good hands given Suntory’s established beverages business, ambitious growth plans and also their recognition of the strong performance and capability of the GSK employees working on these products.”

Shares in GSK dipped 11, or 0.67pc, to £16.40 following confirmation of the deal.

The drinks were owned by Beecham Group prior to its merger in 1989 with SmithKline, and more than three-quarters of their sales are in the UK. They earn annual revenues of about £500m a year. The drinks are also produced in Nigeria, where GSK will continue to manufacture Lucozade and Ribena under licence from Suntory.

GSK shareholders have long questioned the position of the drinks brands within the drugs company. However, when Sir Andrew Witty became chief executive in 2008, he initially backed the consumer healthcare division, with the company talking up the prospects of expanding Lucozade into emerging markets.

GSK kicked off a strategic review of the drink brands earlier this year, which led to the decision to press ahead with a sale.

The deal with Suntory is expected to be completed by the end of the year and GSK said it will use the proceeds “to reduce debt and for general corporate purposes”.

Lucozade is the UK’s top-selling energy and sports drink, while blackcurrant drink Ribena is the fourth-best selling fruit juice and concentrate.

The drinks are the latest British food brands to be sold to foreign investors after deals in the last year for Weetabix, Hartley's jam, Hula Hoops and Sarson’s vinegar.

The deal to buy Lucozade and Ribena, which also attracted interest from the Irn-Bru maker AG Barr, gives Suntory a foothold in the UK market and brands with potential in emerging markets.